Gas sensor

ABSTRACT

A gas sensor includes reference gas regulating device that applies a control voltage that is set between the reference electrode and the measurement-object gas side electrode; and a detecting device that detects the specific gas concentration in the measurement-object gas on the basis of a voltage between the reference electrode and the measurement electrode during a second period, from among a first period that is started upon setting of the control voltage to on state, during which a potential difference between the reference electrode and the measurement-object gas side electrode is large, and the second period that is started upon setting of the control voltage to off state after the potential difference falls from the first period. Tf, a fall time of the potential difference between the first period and the second period, and T2, a second time that is a length of the second period, satisfies Tf≤T2.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a gas sensor.

2. Description of the Related Art

A conventionally known gas sensor detects a specific gas concentrationsuch as NOx in a measurement-object gas such as an exhaust gas of anautomobile. For example, PTL 1 describes a gas sensor including alayered body, a reference electrode, a measurement electrode, and ameasurement-object gas side electrode. The layered body is formed bystacking a plurality of oxygen ion-conductive solid electrolyte layers.The reference electrode is formed inside of the layered body andreceives a reference gas (e.g., air) introduced therein via a referencegas introducing space. The measurement electrode is provided in ameasurement-object gas flowing portion inside the layered body. Themeasurement-object gas side electrode is provided in a region of thelayered body exposed to the measurement-object gas. The gas sensordetects the specific gas concentration in the measurement-object gas onthe basis of an electromotive force generated between the referenceelectrode and the measurement electrode. In addition, the gas sensorfurther includes a reference gas regulating device that pumps in oxygento a periphery of the reference electrode by a flow of control currentby applying a voltage between the reference electrode and themeasurement-object gas side electrode. PTL 1 describes that thereference gas regulating device pumps in oxygen to the periphery of thereference electrode so as to compensate for reduction of the oxygenconcentration caused in a case of temporary reduction of the oxygenconcentration in the reference gas in the periphery of the referenceelectrode and to suppress a decrease of the detection accuracy of thespecific gas concentration. Note that a case of reduction of the oxygenconcentration in the reference gas in the periphery of the referenceelectrode is a case in which, for example, the measurement-object gasslightly enters the reference gas introducing space.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No.2015-200643

SUMMARY OF THE INVENTION

However, in a case in which oxygen in the periphery of themeasurement-object gas side electrode is pumped in to the periphery ofthe reference electrode, voltage is applied between the electrodes toflow control current, thereby changing the potential of the referenceelectrode depending on the applied voltage. Thus, the detection accuracyof the specific gas concentration may be decreased in some cases. Forexample, a change of the potential of the reference electrode alsochanges the voltage between the reference electrode and the measurementelectrode, and thus, the detection accuracy of the specific gasconcentration detected on the basis of the voltage may be decreased insome cases.

The present invention has been made to solve such problems, and its mainobject is to suppress a decrease of the detection accuracy of thespecific gas concentration caused by a pump-in control voltage whilepumping in oxygen to the periphery of the reference electrode.

In order to achieve the above main object, the present invention isconfigured as follows.

A gas sensor according to the present invention comprises

a layered body that is formed by stacking a plurality of oxygenion-conductive solid electrolyte layers, and that includes ameasurement-object gas flowing portion which a measurement-object gas isintroduced and flowed in;

a reference electrode that is formed inside of the layered body, andthat receives a reference gas introduced therein, the reference gasbeing used as a standard for detection of a specific gas concentrationin the measurement-object gas;

a measurement electrode provided on an inner peripheral surface of themeasurement-object gas flowing portion;

a measurement-object gas side electrode provided in a region of thelayered body that is exposed to the measurement-object gas;

a reference gas introducing portion that introduces the reference gas toa periphery of the reference electrode;

a reference gas regulating device that applies a control voltage that isrepetitively set to on state and off state between the referenceelectrode and the measurement-object gas side electrode to pump inoxygen to the periphery of the reference electrode; and

a detecting device that detects the specific gas concentration in themeasurement-object gas on the basis of a voltage between the referenceelectrode and the measurement electrode during a second period, fromamong a first period that is started upon setting of the control voltageto on state, during which a potential difference between the referenceelectrode and the measurement-object gas side electrode is large, andthe second period that is started upon setting of the control voltage tooff state after the potential difference falls from the first period,wherein the gas sensor satisfies the following Formula (1).

Tf≤T2  (1)

(where Tf is a fall time [msec] of the potential difference between thefirst period and the second period, and T2 is a second time [msec] thatis a length of the second period).

In this gas sensor, the reference gas regulating device applies thecontrol voltage between the reference electrode and themeasurement-object gas side electrode to pump in oxygen to the peripheryof the reference electrode. Thus, it is possible to compensate forreduction of the oxygen concentration in the periphery of the referenceelectrode. In addition, since the reference gas regulating deviceapplies the control voltage that is repetitively set to on state and offstate, the gas sensor has the first period during which the potentialdifference between the reference electrode and the measurement-objectgas side electrode is large and the second period that is a period afterthe potential difference between the reference electrode and themeasurement-object gas side electrode falls. Since the control voltageless affects the potential of the reference electrode during the secondperiod than during the first period, by the detecting device detectingthe specific gas concentration on the basis of the voltage between thereference electrode and the measurement electrode during the secondperiod, a decrease of the detection accuracy of the specific gasconcentration can be suppressed. In addition, for example, owing to acapacitance component of the reference electrode or the like, a residualvoltage resulting from the control voltage may be present between thereference electrode and the measurement-object gas side electrode evenduring the second period. This residual voltage affects the potential ofthe reference electrode. Thus, it is likely that the detection accuracyof the specific gas concentration is increased as the residual voltageis lower. Furthermore, since the gas sensor of the present inventionsatisfies Formula (1) (i.e., the ratio T2/Tf is not smaller than 1), thesecond time T2 is made relatively long, and it is possible tosufficiently decrease the residual voltage during the second period.This makes it easier to detect the specific gas concentration with highaccuracy during the second period. Accordingly, this gas sensor cansuppress a decrease in the detection accuracy of the specific gasconcentration caused by the pump-in control voltage while pumping inoxygen to the periphery of the reference electrode. Note that the ratioT2/Tf may be not smaller than 2, or not smaller than 3. The ratio T2/Tfmay be not greater than 6.

The fall time Tf is a time that is necessary for the potentialdifference between the reference electrode and the measurement-objectgas side electrode generated by setting the control voltage to on stateand off state to fall from 90% to 10% where a difference of thepotential difference between its maximum and its minimum is 100%. Thestart of the second period is a timing at which the potential differencefalls to 10%. The end of the second period is a timing at which thepotential difference starts to rise upon the control voltage being setto on state after the start of the second period. The phrase “thespecific gas concentration in the measurement-object gas is detected onthe basis of the voltage between the reference electrode and themeasurement electrode during the second period” includes a case in whichat least a part of a period for detection of the specific gasconcentration slightly deviates from the second period in a range inwhich the above-described effect can be obtained.

In the gas sensor of the present invention, the peak current Ip3maxflowed in the reference electrode by using the control voltage may benot lower than 10 μA. Note that it is likely that the average of thecurrent flowed in the reference electrode by using the control voltagethat is repetitively set to on state and off state is increased as thepeak current Ip3max flowed in the reference electrode by using thecontrol voltage is higher. In addition, the higher the average of thecurrent flowed in the reference electrode, the more effectivelyreduction of the oxygen concentration in the periphery of the referenceelectrode is compensated for. As long as the peak current Ip3max is notlower than 10 μA, it is likely that an effect of compensating forreduction of the oxygen concentration in the periphery of the referenceelectrode is sufficient. The peak current Ip3max may be 150 μA or less.

In the gas sensor of the present invention, the second time T2 may be 10msec or less. Note that the average of the current flowed in thereference electrode by using the control voltage that is repetitivelyset to on state and off state is decreased as the second time T2 islonger. In addition, if the average of the current flowed in thereference electrode is low, it is likely that the effect of compensatingfor reduction of the oxygen concentration in the periphery of thereference electrode becomes insufficient. As long as the second time T2is not longer than 10 msec, it is likely that insufficiency of theeffect of compensating for reduction of the oxygen concentration in theperiphery of the reference electrode is suppressed.

In the gas sensor of the present invention, the fall time Tf may be notlonger than 3 msec. Note that it is likely that the residual voltage isdecreased more rapidly during the second period as the fall time Tf isshorter. As long as the fall time Tf is 3 msec or less, it is likelythat the second time T2 is made relatively short or the residual voltageis sufficiently decreased during the second period while maintaining thedetection accuracy of the specific gas concentration.

In the gas sensor of the present invention, a fall residual voltageDVref10 calculated according to the following Formula (2) may be 55 mVor less. If the fall residual voltage DVref10 of the gas sensor measuredin the air is 55 mV or less, the fall residual voltage in themeasurement-object gas is sufficiently decreased. The lower the fallresidual voltage, the lower the residual voltage during the secondperiod, and thus, it is likely that the specific gas concentration isdetected with high accuracy during the second period.

DVref10=(Vref2−Vref1)×0.1+Vref1−Vref0   (2),

(where Vref0 is a voltage [mV] between the reference electrode and themeasurement-object gas side electrode in a state in which the layeredbody is placed in an air and in which the control voltage is notapplied,

Vref1 is a minimum voltage [mV] between the reference electrode and themeasurement-object gas side electrode in a state in which the layeredbody is placed in the air and in which the control voltage isrepetitively set to on state and off state, and

Vref2 is a maximum voltage [mV] between the reference electrode and themeasurement-object gas side electrode in a state in which the layeredbody is placed in the air and in which the control voltage isrepetitively set to on state and off state.)

In the sensor element of the present invention, the measurement-objectgas side electrode may be provided on the outer surface of the layeredbody. In the gas sensor of the present invention, the sensor element mayinclude an outer electrode provided on the outer surface of the layeredbody. In addition, the detecting device may pump in or pump out oxygenvia the measurement electrode and the outer electrode on the basis ofthe voltage between the reference electrode and the measurementelectrode and may detect the specific gas concentration in themeasurement-object gas on the basis of the current at the time ofpumping in or pumping out. In this case, the outer electrode may serveas the measurement-object gas side electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of a gas sensor 100.

FIG. 2 is a sectional schematic diagram schematically illustrating anexample of a configuration of a sensor element 101.

FIG. 3 is an explanatory diagram illustrating a temporal change of acontrol voltage Vp3 and a voltage Vref.

FIG. 4 is an explanatory diagram of a fall residual voltage DVref10.

FIG. 5 is a sectional schematic diagram of a sensor element 201according to a modification.

DETAILED DESCRIPTION OF THE INVENTION

Next, an embodiment of the present invention will be described withreference to the drawings. FIG. 1 is a vertical sectional view of a gassensor 100, which is an embodiment of the present invention. FIG. 2 is asectional schematic diagram schematically illustrating an example of aconfiguration of a sensor element 101 included in the gas sensor 100.The sensor element 101 is in a long rectangular parallelepiped shape. Inthe description below, a longitudinal direction of the sensor element101 (left-right direction in FIG. 2) is a front-rear direction, and athickness direction of the sensor element 101 (vertical direction inFIG. 2) is a vertical direction. A width direction of the sensor element101 (direction perpendicular to the front-rear direction and thevertical direction) is a left-right direction.

As illustrated in FIG. 1, the gas sensor 100 has the sensor element 101,a protective cover 130 configured to protect a front end side of thesensor element 101, and a sensor assembly 140 including a connector 150that has continuity with the sensor element 101. This gas sensor 100 ismounted to, for example, a piping 190 such as an exhaust gas pipe of avehicle as illustrated and is used to measure a specific gasconcentration such as NOx or O₂ included in an exhaust gas which is ameasurement-object gas. According to this embodiment, the gas sensor 100is used to measure the NOx concentration as the specific gasconcentration.

The protective cover 130 includes an inner protective cover 131 in abottomed cylindrical shape to cover a front end of the sensor element101 and an outer protective cover 132 in a bottomed cylindrical shape tocover the inner protective cover 131. The inner protective cover 131 andthe outer protective cover 132 have a plurality of holes formed to flowthe measurement-object gas inside of the protective cover 130. A sensorelement chamber 133 is formed as a space surrounded by the innerprotective cover 131. The front end of the sensor element 101 is placedin the sensor element chamber 133.

The sensor assembly 140 includes an element sealed body 141 in which thesensor element 101 is sealed and fixed, a nut 147 mounted to the elementsealed body 141, an outer cylinder 148, and the connector 150 that is incontact with and is electrically connected with not shown connectorelectrodes (only a heater connector electrode 71, which will bedescribed later, is shown in FIG. 2) formed on surfaces (upper and lowersurfaces) at a rear end of the sensor element 101.

The element sealed body 141 includes a main fitting 142 in a cylindricalshape, an inner cylinder 143 in a cylindrical shape coaxially welded andfixed to the main fitting 142, ceramic supporters 144 a to 144 c sealedin through holes inside of the main fitting 142 and the inner cylinder143, green compacts 145 a and 145 b, and a metal ring 146. The sensorelement 101 is located on a center axis of the element sealed body 141to pass through the element sealed body 141 in the front-rear direction.The inner cylinder 143 has a reduced diameter portion 143 a formed topress the green compact 145 b in a direction of the center axis of theinner cylinder 143, and a reduced diameter portion 143 b formed to pressforward the ceramic supporters 144 a to 144 c and the green compacts 145a and 145 b via the metal ring 146. The pressing force from the reduceddiameter portions 143 a and 143 b causes the green compacts 145 a and145 b to be compressed between the main fitting 142 or the innercylinder 143 and the sensor element 101. The green compacts 145 a and145 b accordingly seal the sensor element chamber 133 in the protectivecover 130 from a space 149 in the outer cylinder 148, while fixing thesensor element 101.

The nut 147 is coaxially fixed to the main fitting 142 and has a malethreaded portion formed on its outer peripheral surface. The malethreaded portion of the nut 147 is inserted into a fixation member 191that is welded to the piping 190 and is formed to have a female threadedportion on its inner peripheral surface. The gas sensor 100 isaccordingly fixed to the piping 190 in a state in which the front end ofthe sensor element 101 and the protective cover 130 of the gas sensor100 are protruded into the piping 190.

The outer cylinder 148 is provided to cover a periphery of the innercylinder 143, the sensor element 101, and the connector 150. A pluralityof lead wires 155 connected with the connector 150 are drawn outsidefrom a rear end of the outer cylinder 148. The lead wires 155 areelectrically connected with respective electrodes (described later) ofthe sensor element 101 via the connector 150. A clearance between theouter cylinder 148 and the lead wires 155 is sealed by a rubber plug157. The space 149 in the outer cylinder 148 is filled with a referencegas (the air in the embodiment). The rear end of the sensor element 101is placed in the space 149.

The sensor element 101 is an element of a layered body in which sixlayers, respectively made of an oxygen ion-conductive solid electrolytesuch as zirconia (ZrO₂), including a first substrate layer 1, a secondsubstrate layer 2, a third substrate layer 3, a first solid electrolytelayer 4, a spacer layer 5, and a second solid electrolyte layer 6 arestacked in this sequence from a lower side of the drawing. The solidelectrolyte forming these six layers is dense and air-tight. The sensorelement 101 of this configuration may be manufactured, for example, bymaking ceramic green sheets corresponding to the respective layerssubjected to, for example, predetermined processing and printing of acircuit pattern, stacking the processed green sheets, and firing thestacked green sheets to be integrated.

A gas inlet port 10, a first diffusion controlling portion 11, a bufferspace 12, a second diffusion controlling portion 13, a first internalcavity 20, a third diffusion controlling portion 30, a second internalcavity 40, a fourth diffusion controlling portion 60, and a thirdinternal cavity 61 are formed to be adjacent to one another andcommunicate with one another in this sequence on one end (left end inFIG. 2) of the sensor element 101 and between a lower surface of thesecond solid electrolyte layer 6 and an upper surface of the first solidelectrolyte layer 4.

The gas inlet port 10, the buffer space 12, the first internal cavity20, the second internal cavity 40, and the third internal cavity 61 areformed as internal spaces of the sensor element 101 by cutting out thespacer layer 5 to have an upper portion defined by the lower surface ofthe second solid electrolyte layer 6, a lower portion defined by theupper surface of the first solid electrolyte layer 4, and a side portiondefined by a side surface of the spacer layer 5.

Each of the first diffusion controlling portion 11, the second diffusioncontrolling portion 13, and the third diffusion controlling portion 30is provided in the form of two horizontally long slits (where alongitudinal direction of their openings is a direction perpendicular tothe sheet surface). The fourth diffusion controlling portion 60 isprovided in the form of one horizontally long slit (where a longitudinaldirection of their openings is a direction perpendicular to the sheetsurface) that is formed as a clearance from the lower surface of thesecond solid electrolyte layer 6. A region from the gas inlet port 10 tothe third internal cavity 61 is also called a measurement-object gasflowing portion.

An air introducing layer 48 is provided between an upper surface of thethird substrate layer 3 and a lower surface of the first solidelectrolyte layer 4. The air introducing layer 48 is, for example, madeof a ceramic porous material such as alumina. The air introducing layer48 has a rear end surface serving as an inlet portion 48 c, and theinlet portion 48 c is exposed to the rear end surface of the sensorelement 101. The inlet portion 48 c is exposed to the space 149illustrated in FIG. 1 (see FIG. 1). The reference gas for measuring theNOx concentration is introduced through the inlet portion 48 c into theair introducing layer 48. The reference gas is the air (atmosphere inthe space 149 in FIG. 1) in this embodiment. In addition, the airintroducing layer 48 is formed to cover a reference electrode 42. Theair introducing layer 48 applies a predetermined diffusion resistance tothe reference gas introduced from the inlet portion 48 c and introducesthe resistance-applied reference gas into the reference electrode 42.The air introducing layer 48 may have a thickness of 10 μm or more and30 μm or less. The air introducing layer 48 may have a porosity of 10volume % or more and 50 volume % or less.

The reference electrode 42 is an electrode formed between the uppersurface of the third substrate layer 3 and the first solid electrolytelayer 4. The air introducing layer 48 is provided in the periphery ofthe reference electrode 42 as described above. The reference electrode42 is formed directly on the upper surface of the third substrate layer3, and a remaining part of the reference electrode 42 other than thepart in contact with the upper surface of the third substrate layer 3 iscovered by the air introducing layer 48. However, at least a part of thereference electrode 42 has to be covered by the air introducing layer48. In addition, as will be described later, the oxygen concentrations(oxygen partial pressures) in the first internal cavity 20, in thesecond internal cavity 40, and in the third internal cavity 61 aremeasurable by using the reference electrode 42. The reference electrode42 is formed as a porous cermet electrode (for example, cermet electrodeof Pt and ZrO₂).

In the measurement-object gas flowing portion, the gas inlet port 10 isa region open to an external space and is arranged such that themeasurement-object gas is taken from the external space through the gasinlet port 10 into the sensor element 101. The first diffusioncontrolling portion 11 is a region that applies a predetermineddiffusion resistance to the measurement-object gas taken from the gasinlet port 10. The buffer space 12 is a space provided to lead themeasurement-object gas that is introduced from the first diffusioncontrolling portion 11, to the second diffusion controlling portion 13.The second diffusion controlling portion 13 is a region that applies apredetermined diffusion resistance to the measurement-object gas that isintroduced from the buffer space 12 into the first internal cavity 20.In the course of introducing the measurement-object gas from outside ofthe sensor element 101 into the first internal cavity 20, themeasurement-object gas rapidly taken from the gas inlet port 10 into thesensor element 101 by a pressure variation of the measurement-object gasin the external space (pulsation of exhaust gas pressure in a case inwhich the measurement-object gas is an exhaust gas of an automobile) isnot directly introduced into the first internal cavity 20 but isintroduced into the first internal cavity 20 after cancellation of aconcentration variation of the measurement-object gas through the firstdiffusion controlling portion 11, the buffer space 12, and the seconddiffusion controlling portion 13. This reduces the concentrationvariation of the measurement-object gas introduced into the firstinternal cavity 20 to a substantially negligible level. The firstinternal cavity 20 is provided as a space to regulate the oxygen partialpressure in the measurement-object gas introduced through the seconddiffusion controlling portion 13. The oxygen partial pressure isregulated by operation of a main pump cell 21.

The main pump cell 21 is an electrochemical pump cell, which includes aninner pump electrode 22 having a top electrode portion 22 a providedover a substantially entire lower surface of the second solidelectrolyte layer 6 facing the first internal cavity 20, an outer pumpelectrode 23 provided in a region corresponding to the top electrodeportion 22 a on an upper surface of the second solid electrolyte layer 6to be exposed to an external space (sensor element chamber 133 in FIG.1), and the second solid electrolyte layer 6 placed between theseelectrodes 22 and 23.

The inner pump electrode 22 is formed across the upper and lower solidelectrolyte layers (the second solid electrolyte layer 6 and the firstsolid electrolyte layer 4) defining the first internal cavity 20 and thespacer layer 5 forming the side wall. Specifically, the top electrodeportion 22 a is formed on the lower surface of the second solidelectrolyte layer 6 that forms a top surface of the first internalcavity 20. A bottom electrode portion 22 b is formed directly on theupper surface of the first solid electrolyte layer 4 that forms a bottomsurface of the first internal cavity 20. Side electrode portions(omitted from illustrations) are formed on side wall surfaces (innersurfaces) of the spacer layer 5 that form both side wall portions of thefirst internal cavity 20, such as to connect the top electrode portion22 a with the bottom electrode portion 22 b and provide a tunnel-likestructure in the region where the side electrode portions are provided.

The inner pump electrode 22 and the outer pump electrode 23 are formedas porous cermet electrodes (for example, cermet electrodes of Pt andZrO₂ containing 1% Au). The inner pump electrode 22 in contact with themeasurement-object gas is made of a material having the decreasedreducing ability with regard to the NOx component in themeasurement-object gas.

The main pump cell 21 is capable of pumping out oxygen from the firstinternal cavity 20 to the external space or pumping in oxygen from theexternal space to the first internal cavity 20 by applying a desiredpump voltage Vp0 between the inner pump electrode 22 and the outer pumpelectrode 23 and making a pump current Ip0 flow in a positive directionor a negative direction between the inner pump electrode 22 and theouter pump electrode 23.

In order to detect the oxygen concentration (oxygen partial pressure) inthe atmosphere of the first internal cavity 20, the inner pump electrode22, the second solid electrolyte layer 6, the spacer layer 5, the firstsolid electrolyte layer 4, and the reference electrode 42 constitute anelectrochemical sensor cell, that is, a main pump-controlling oxygenpartial pressure detection sensor cell 80.

The oxygen concentration (oxygen partial pressure) in the first internalcavity 20 is determined by measuring an electromotive force V0 in themain pump-controlling oxygen partial pressure detection sensor cell 80.The pump current Ip0 is controlled by feedback control of the pumpvoltage Vp0 of a variable power supply 25 to keep the electromotiveforce V0 constant. This maintains the oxygen concentration in the firstinternal cavity 20 at a predetermined constant value.

The third diffusion controlling portion 30 is a region that applies apredetermined diffusion resistance to the measurement-object gas withthe oxygen concentration (oxygen partial pressure) controlled byoperation of the main pump cell 21 in the first internal cavity 20 andleads the resistance-applied measurement-object gas to the secondinternal cavity 40.

The second internal cavity 40 is provided as a space to further regulatethe oxygen partial pressure by means of an auxiliary pump cell 50 withrespect to the measurement-object gas introduced through the thirddiffusion controlling portion 30 after regulation of the oxygenconcentration (oxygen partial pressure) in the first internal cavity 20.This maintains the oxygen concentration in the second internal cavity 40constant with high accuracy and thus enables the gas sensor 100 tomeasure the NOx concentration with high accuracy.

The auxiliary pump cell 50 is an auxiliary electrochemical pump cell,which includes an auxiliary pump electrode 51 having a top electrodeportion 51 a provided over a substantially entire lower surface of thesecond solid electrolyte layer 6 facing the second internal cavity 40,the outer pump electrode 23 (or any appropriate electrode outside of thesensor element 101 in place of the outer pump electrode 23), and thesecond solid electrolyte layer 6.

The auxiliary pump electrode 51 is provided to have a tunnel-likestructure like the inner pump electrode 22 provided in the firstinternal cavity 20 and is placed in the second internal cavity 40. Thatis, the top electrode portion 51 a is formed on the second solidelectrolyte layer 6 that forms a top surface of the second internalcavity 40. A bottom electrode portion 51 b is formed directly on theupper surface of the first solid electrolyte layer 4 that forms a bottomsurface of the second internal cavity 40. Side electrode portions(omitted from illustrations) are formed on side wall surfaces of thespacer layer 5 that form side walls of the second internal cavity 40,such as to connect the top electrode portion 51 a with the bottomelectrode portion 51 b and provide a tunnel-like structure. Like theinner pump electrode 22, the auxiliary pump electrode 51 is made of amaterial having the decreased reducing ability with regard to the NOxcomponent in the measurement-object gas.

The auxiliary pump cell 50 is capable of pumping out oxygen in theatmosphere from the second internal cavity 40 to the external space orpumping in oxygen from the external space to the second internal cavity40 by applying a desired voltage Vp1 between the auxiliary pumpelectrode 51 and the outer pump electrode 23.

In order to control the oxygen partial pressure in the atmosphere of thesecond internal cavity 40, the auxiliary pump electrode 51, thereference electrode 42, the second solid electrolyte layer 6, the spacerlayer 5, and the first solid electrolyte layer 4 constitute anelectrochemical sensor cell, that is, an auxiliary pump-controllingoxygen partial pressure detection sensor cell 81.

The auxiliary pump cell 50 performs pumping at a variable power supply52 under voltage control based on an electromotive force V1 detected bythe auxiliary pump-controlling oxygen partial pressure detection sensorcell 81. This controls the oxygen partial pressure in the atmosphere ofthe second internal cavity 40 to such a low partial pressure thatsubstantially does not affect the measurement of NOx.

Additionally, its pump current Ip1 is used to control the electromotiveforce of the main pump-controlling oxygen partial pressure detectionsensor cell 80.

Specifically, the pump current Ip1 is input as a control signal into themain pump-controlling oxygen partial pressure detection sensor cell 80to control its electromotive force V0. This control maintains a constantslope of the oxygen partial pressure in the measurement-object gas thatis introduced from the third diffusion controlling portion 30 into thesecond internal cavity 40. In a case in which the gas sensor 100 is usedas a NOx sensor, the oxygen concentration in the second internal cavity40 is maintained at a constant level of approximately 0.001 ppm by theoperation of the main pump cell 21 and the auxiliary pump cell 50.

The fourth diffusion controlling portion 60 is a region that applies apredetermined diffusion resistance to the measurement-object gas withthe oxygen concentration (oxygen partial pressure) controlled byoperation of the auxiliary pump cell 50 in the second internal cavity 40and leads the resistance-applied measurement-object gas to the thirdinternal cavity 61. The fourth diffusion controlling portion 60 servesto limit the amount of NOx flowing into the third internal cavity 61.

The third internal cavity 61 is provided as a space to further perform aprocess on the measurement-object gas introduced through the fourthdiffusion controlling portion 60 after regulation of the oxygenconcentration (oxygen partial pressure) in the second internal cavity40, the process being related to measurement of the concentration ofnitrogen oxides (NOx) in the measurement-object gas. Measurement of theNOx concentration is mainly performed in the third internal cavity 61 byoperation of a measurement pump cell 41.

The measurement pump cell 41 measures the NOx concentration in themeasurement-object gas in the third internal cavity 61. The measurementpump cell 41 is an electrochemical pump cell, which includes ameasurement electrode 44 provided directly on an upper surface of thefirst solid electrolyte layer 4 facing the third internal cavity 61, theouter pump electrode 23, the second solid electrolyte layer 6, thespacer layer 5, and the first solid electrolyte layer 4. The measurementelectrode 44 is a porous cermet electrode. The measurement electrode 44also serves as a NOx reducing catalyst to reduce NOx present in theatmosphere of the third internal cavity 61.

The measurement pump cell 41 is capable of pumping out oxygen producedby degradation of nitrogen oxides in the ambient atmosphere of themeasurement electrode 44 and detecting the production amount of oxygenas a pump current Ip2.

In order to detect the oxygen partial pressure in the periphery of themeasurement electrode 44, the first solid electrolyte layer 4, themeasurement electrode 44, and the reference electrode 42 constitute anelectrochemical sensor cell, that is, a measurement pump-controllingoxygen partial pressure detection sensor cell 82. A variable powersupply 46 is controlled on the basis of an electromotive force (voltageV2) detected by the measurement pump-controlling oxygen partial pressuredetection sensor cell 82.

The measurement-object gas introduced into the second internal cavity 40passes through the fourth diffusion controlling portion 60 with theoxygen partial pressure controlled and reaches the measurement electrode44 in the third internal cavity 61. Nitrogen oxides in themeasurement-object gas in the periphery of the measurement electrode 44are reduced to produce oxygen (2NO→N₂+O₂). The produced oxygen issubjected to pumping by the measurement pump cell 41. In this process, avoltage Vp2 of the variable power supply 46 is controlled to maintainconstant the voltage V2 detected by the measurement pump-controllingoxygen partial pressure detection sensor cell 82. Since the amount ofoxygen produced in the periphery of the measurement electrode 44 isproportional to the concentration of nitrogen oxides in themeasurement-object gas, the concentration of nitrogen oxides in themeasurement-object gas is calculated by using the pump current Ip2 ofthe measurement pump cell 41.

The second solid electrolyte layer 6, the spacer layer 5, the firstsolid electrolyte layer 4, the third substrate layer 3, the outer pumpelectrode 23, and the reference electrode 42 constitute anelectrochemical sensor cell 83. The oxygen partial pressure in themeasurement-object gas outside of the sensor is detectable by using anelectromotive force (voltage Vref) obtained by this sensor cell 83.

Additionally, the second solid electrolyte layer 6, the spacer layer 5,the first solid electrolyte layer 4, the third substrate layer 3, theouter pump electrode 23, and the reference electrode 42 constitute anelectrochemical reference gas regulation pump cell 90. The reference gasregulation pump cell 90 performs pumping by means of a control currentIp3 flowed by using a control voltage Vp3 applied by a power supplycircuit 92 connected between the outer pump electrode 23 and thereference electrode 42. The reference gas regulation pump cell 90accordingly pumps in oxygen from an ambient space of the outer pumpelectrode 23 (sensor element chamber 133 in FIG. 1) to an ambient spaceof the reference electrode 42 (air introducing layer 48).

In the gas sensor 100 having the above configuration, the measurementpump cell 41 receives the measurement-object gas with the oxygen partialpressure maintained at a constant low value (value that does notsubstantially affect the measurement of NOx) by the operation of themain pump cell 21 and the auxiliary pump cell 50. Accordingly, the NOxconcentration in the measurement-object gas is determinable, on thebasis of the pump current Ip2 flowed by the measurement pump cell 41pumping out oxygen produced by reduction of NOx approximately inproportion to the concentration of NOx in the measurement-object gas.

Additionally, the sensor element 101 is provided with a heater unit 70serving to adjust a temperature to heat the sensor element 101 and keepthe sensor element 101 warm, in order to enhance the oxygen ionconductivity of the solid electrolyte. The heater unit 70 includes aheater connector electrode 71, a heater 72, a through hole 73, a heaterinsulating layer 74, a pressure release hole 75, and a lead wire 76.

The heater connector electrode 71 is an electrode formed to be incontact with a lower surface of the first substrate layer 1. Connectingthe heater connector electrode 71 with an external power supply allowsfor external power feeding to the heater unit 70.

The heater 72 is an electric resistor formed to be placed between thesecond substrate layer 2 and the third substrate layer 3. The heater 72is connected with the heater connector electrode 71 via the lead wire 76and the through hole 73 and generates heat by external power feedingthrough the heater connector electrode 71 to heat the solid electrolyteincluded in the sensor element 101 and keep the solid electrolyte warm.

The heater 72 is embedded over an entire area from the first internalcavity 20 to the third internal cavity 61 and is capable of adjustingthe entire sensor element 101 to a temperature at which the solidelectrolyte is activated.

The heater insulating layer 74 is an insulating layer of porous aluminaformed from an insulating material such as alumina on upper and lowersurfaces of the heater 72. The heater insulating layer 74 is formed toprovide electrical insulation between the second substrate layer 2 andthe heater 72 and electrical insulation between the third substratelayer 3 and the heater 72.

The pressure release hole 75 is a region provided to pass through thethird substrate layer 3 and the air introducing layer 48 and is formedto relieve an increase in internal pressure accompanied with atemperature rise in the heater insulating layer 74.

The variable power supplies 25, 46, and 52 and the power supply circuit92 illustrated in FIG. 2 and the like are actually connected with therespective electrodes via lead wires (not shown) formed in the sensorelement 101 and the connector 150 and the lead wires 155 illustrated inFIG. 1.

The following describes an example of a manufacturing method of the gassensor 100. First, six unfired ceramic green sheets are prepared, eachcontaining an oxygen ion-conductive solid electrolyte such as zirconiaas the ceramic component. A plurality of sheet holes used forpositioning in printing or in stacking, a plurality of required throughholes, and the like are formed in advance in the respective greensheets. A space forming the measurement-object gas flowing portion isprovided in advance by, for example, punching in the green sheet for thespacer layer 5. Subsequently, a pattern printing process and a dryingprocess are performed to form various patterns in the respective ceramicgreen sheets respectively corresponding to the first substrate layer 1,the second substrate layer 2, the third substrate layer 3, the firstsolid electrolyte layer 4, the spacer layer 5, and the second solidelectrolyte layer 6. Specifically, the patterns formed include, forexample, the respective electrodes described above, lead wiresconnecting with the respective electrodes, the air introducing layer 48,and the heater unit 70. The pattern printing is performed by applyingpattern-forming paste provided according to the properties required foreach object on the green sheet by a known screen printing technique. Thedrying process also employs any known drying technique. On completion ofpattern printing and drying, the procedure performs a printing anddrying process to print and dry an adhesive paste for stacking andbonding the green sheets corresponding to the respective layers. Theprocedure then performs a press bonding process to position therespective green sheets with the adhesive paste by aligning the sheetholes, stack the respective green sheets in a predetermined sequence,and pressure bond the respective green sheets under predeterminedtemperature and pressure conditions to form one layered body. Theresulting layered body includes a plurality of sensor elements 101. Thelayered body is cut into the size of the sensor elements 101. Each ofthe cut-out piece of the layered body is fired at a predetermined firingtemperature to provide the sensor element 101.

After obtaining the sensor element 101, the procedure produces thesensor assembly 140 (see FIG. 1) with the sensor element 101 builttherein and mounts the components such as the protective cover 130 andthe rubber plug 157 to the sensor assembly 140 to complete the gassensor 100.

The following describes the functions of the reference gas regulationpump cell 90 in detail. The measurement-object gas is introduced fromthe sensor element chamber 133 illustrated in FIG. 1 to themeasurement-object gas flowing portion of the sensor element 101including, for example, the gas inlet port 10. The reference gas (air)in the space 149 illustrated in FIG. 1 is, on the other hand, introducedinto the air introducing layer 48 of the sensor element 101. The sensorelement chamber 133 and the space 149 are separated from each other bythe sensor assembly 140 (especially the green compacts 145 a and 145 b)and are sealed to prevent the gas from flowing therebetween. When thepressure of the measurement-object gas is temporarily increased,however, the measurement-object gas may slightly enter the space 149.This causes temporary reduction of the oxygen concentration in theperiphery of the reference electrode 42 and thereby results in changingthe reference potential that is the potential of the reference electrode42. This may change a voltage based on the reference electrode 42, forexample, the voltage V2 of the measurement pump-controlling oxygenpartial pressure detection sensor cell 82 and decrease the detectionaccuracy of the NOx concentration in the measurement-object gas. Thereference gas regulation pump cell 90 serves to suppress such a decreaseof the detection accuracy. The reference gas regulation pump cell 90pumps in oxygen from the periphery of the outer pump electrode 23 to theperiphery of the reference electrode 42 by the flow of the controlcurrent Ip3 by applying the control voltage Vp3 between the referenceelectrode 42 and the outer pump electrode 23. As described above, thiscompensates for reduction of oxygen and suppresses a decrease of thedetection accuracy of the NOx concentration when the measurement-objectgas has temporarily reduced the oxygen concentration in the periphery ofthe reference electrode 42.

The power supply circuit 92 of the reference gas regulation pump cell 90applies a voltage that is repetitively set to on state and off state asthe control voltage Vp3. Thus, there are a first period during which thevoltage Vref between the reference electrode 42 and the outer pumpelectrode 23 has a large value (=potential difference between thereference electrode 42 and the outer pump electrode 23), and a secondperiod during which the voltage Vref has a small value. FIG. 3 is anexplanatory diagram illustrating a temporal change of the controlvoltage Vp3 and the voltage Vref. The upper part of FIG. 3 illustrates atemporal change of the control voltage Vp3, and the lower partillustrates a temporal change of the voltage Vref. The control voltageVp3 and the voltage Vref are positive if the potential of the referenceelectrode 42 is higher than that of the outer pump electrode 23, and theup direction of the vertical axis in FIG. 3 is a positive direction. Asillustrated in FIG. 3, the control voltage Vp3 is a voltage having apulse waveform that is repetitively set to on state and off state in acycle T. For example, when the control voltage Vp3 is set to on state attime t1, the control voltage Vp3 rises from 0 V to a maximum voltageVp3max, and this state is continued until time t4 at which an on timeTon comes. When the control voltage Vp3 is set to off state at time t4,the control voltage Vp3 remains at 0 V until time t7 at which an offtime Toff comes. With this control voltage Vp3, the voltage Vref startsto rise at time t1 to become a maximum voltage Vrefmax at time t4, andstarts to fall at time t4 to become a minimum voltage Vrefmim at timet7. At this time, a difference between the maximum voltage Vrefmax andthe minimum voltage Vrefmim of the voltage Vref, generated by thecontrol voltage Vp3 set to on state and off state, is determined as100%, and by using this as a standard, a rise period, the first period,a fall period, and the second period of the voltage Vref are determined.Specifically, a period during which the voltage Vref rises from 10% to90% (time t2 to t3) is determined as the rise period, and the lengththereof is determined as a rise time Tr. A period during which thevoltage Vref remains to 90% or more (time t3 to t5) is determined as thefirst period, and the length thereof is determined as a first time T1. Aperiod during which the voltage Vref falls from 90% to 10% (time t5 tot6) is determined as the fall period, and the length thereof isdetermined as a fall time Tf. A period from when the voltage Vref fallsto 10% to when the voltage Vref starts to rise in response to thecontrol voltage Vp3 set to on state in the next cycle (time t6 to t7) isdetermined as the second period, and the length thereof is determined asa second time T2. The voltage Vref at the start of the second period,that is, the voltage obtained when the voltage Vref falls to 10%, isdetermined as a fall voltage V10. Note that the voltage Vref in FIG. 3becomes the maximum voltage Vrefmax for the first time at time t4 atwhich the control voltage Vp3 falls. However, in a case in which the ontime Ton is long, the voltage Vref can reach the maximum voltage Vrefbefore time t4.

The measurement pump cell 41 detects the NOx concentration in themeasurement-object gas on the basis of the voltage V2 during the secondperiod. More specifically, during the second period, the measurementpump cell 41 acquires the value of the voltage V2, performs feedbackcontrol of the voltage Vp2 of the variable power supply 46 so as tomaintain the voltage V2 at a predetermined constant value, and detectsthe value of the pump current Ip2 flowed by using the voltage Vp2. Sincethe measurement pump cell 41 detects the NOx concentration (the pumpcurrent Ip2 here) during the second period in this manner, it ispossible to suppress a decrease of the detection accuracy of the NOxconcentration caused by the control voltage Vp3 used for pumping inoxygen to the measurement electrode 44. For example, a case in which themeasurement pump cell 41 measures the NOx concentration during the firstperiod is considered. In this case, since the control voltage Vp3 is setto on state during the first period, unlike during the second period,the voltage Vref is changed to be higher than a voltage Vref* that is asupposed value (voltage based on an oxygen concentration differencebetween the periphery of the reference electrode 42 and the periphery ofthe outer pump electrode 23). This changes the potential of thereference electrode 42 and also changes the voltage V2. Accordingly, ifthe measurement pump cell 41 flows the pump current Ip2 on the basis ofthe voltage V2 during the first period, it is likely that the pumpcurrent Ip2 deviates from the correct value representing the NOxconcentration and that the detection accuracy of the NOx concentrationis decreased. In contrast, during the second period, the control voltageVp3 less affects the potential of the reference electrode 42 than duringthe first period. Specifically, the voltage Vref during the secondperiod after the voltage Vref has fallen is a value closer to thevoltage Vref* than the voltage Vref during the first period. Thus, bythe measurement pump cell 41 measuring the NOx concentration during thesecond period, a decrease of the detection accuracy of the NOxconcentration can be suppressed.

As seen in FIG. 3, the voltage Vref is decreased by taking a long timefrom the timing at which the control voltage Vp3 is set to off state.This is considered to be resulting from a capacitance component of thereference electrode 42 or the like, for example. Thus, even during thesecond period, a residual voltage Vrs resulting from the control voltageVp3 may be present between the reference electrode 42 and the outer pumpelectrode 23. In this case, for example, the voltage Vref during thesecond period is the sum of the voltage Vref* and the residual voltageVrs. Since the residual voltage Vrs affects the potential of thereference electrode 42, it is likely that the detection accuracy of theNOx concentration is increased as the residual voltage Vrs is decreased.Therefore, the residual voltage Vrs is preferably as low as possible.For example, the fall voltage V10 is preferably as low as possible, andthe minimum voltage Vrefmin is preferably as low as possible. Inaddition, since the residual voltage Vrs is decreased over time alsoduring the second period, it is likely that a decrease of the detectionaccuracy of the NOx concentration is suppressed more effectively as thetime is closer to the end of the second period (t7 in FIG. 3).Accordingly, the measurement pump cell 41 preferably detects the NOxconcentration at a timing as later as possible during the second period.Furthermore, a period required for the measurement pump cell 41 todetect the NOx concentration (e.g., the above-described period from thedetection of the voltage V2 to the detection of the value of the pumpcurrent Ip2) is preferably included in the second period. Themeasurement pump cell 41 preferably detects the NOx concentration in thesame cycle T as the setting of the control voltage Vp3 to on state andoff state. In this manner, it is possible to repetitively detect the NOxconcentration at the same timing during the second period at each cycleT.

In addition, the gas sensor 100 satisfies the following Formula (1).That is, in this embodiment, the second time T2 is made relatively long.

Tf≤T2  (1)

(where Tf is a fall time [msec] of a potential difference between thefirst period and the second period, and

T2 is the second time [msec] that is the length of the second period)

Note that the longer the second time T2, the lower the residual voltageVrs becomes during the second period, and the minimum voltage Vrefmin isalso further decreased. In other words, a difference between the minimumvoltage Vrefmin and the voltage Vref* is reduced. As long as Formula (1)is satisfied, that is, as long as the ratio T2/Tf is 1 or more, theresidual voltage Vrs is sufficiently decreased during the second period,and thus, it is likely that the NOx concentration is detected with highaccuracy during the second period. For example, unless Formula (1) issatisfied, even if the measurement pump cell 41 detects the NOxconcentration at a timing as later as possible during the second period,the detection accuracy of the NOx concentration may be decreased;however, as long as the Formula (1) is satisfied, such a decrease can besuppressed. In addition, the longer the second time T2 (the larger theratio T2/Tf), the lower the minimum voltage Vrefmin. Accordingly, if theNOx concentration is detected at a timing as later as possible duringthe second period, the detection accuracy of the NOx concentration isincreased. Alternatively, the longer the second time T2, the longer theperiod during which the NOx concentration can be detected with highaccuracy during the second period. The ratio T2/Tf may be 2 or more, or3 or more. The ratio T2/Tf may be 6 or less. It is likely that the NOxconcentration is detected with higher accuracy during the second periodas the ratio T2/Tf is larger.

The peak current Ip3max flowed in the reference electrode 42 by usingthe control voltage Vp3 is preferably not lower than 10 JA. The waveformof the control current Ip3 flowed in the reference electrode 42 by usingthe control voltage Vp3 has in principle the same phase as the waveformof the voltage Vref illustrated in FIG. 3. The peak current Ip3max isthe value of the control current Ip3 flowed when the voltage Vrefbecomes the maximum voltage Vrefmax. Note that it is likely that theaverage of the current Ip3 flowed in the reference electrode 42 by usingthe control voltage Vp3 that is repetitively set to on state and offstate (average current in the cycle T) is increased as the peak currentIp3max is higher. In addition, the higher the average of the current Ip3flowed in the reference electrode 42, the more effectively reduction ofthe oxygen concentration in the periphery of the reference electrode 42is compensated for. As long as the peak current Ip3max is 10 μA or more,it is likely that the effect of compensating for reduction of the oxygenconcentration in the periphery of the reference electrode 42 issufficient. The peak current Ip3max may be 5 μA or more, or 10 μA ormore. The peak current Ip3max may be 300 μA or less, or 150 μA or less.The value of the average of the control current Ip3 can be determined inadvance by experiment or the like on the basis of how much the oxygenconcentration in the periphery of the reference electrode 42 isdecreased (how much oxygen needs to be pumped in to the periphery of thereference electrode 42) when the pressure of the measurement-object gasis an assumable maximum. Therefore, the peak current Ip3max may bedetermined by taking into account the thus determined average of thecontrol current Ip3.

The higher the average of the current Ip3 flowed in the referenceelectrode 42 (average current in the cycle T), the more likely it isthat degradation occurs, such as fining of grains constituting thereference electrode 42, and thereby it is more likely that theresistance of the reference electrode 42 is increased in long time use.Once the resistance of the reference electrode 42 is increased, thereference potential that is the potential of the reference electrode 42is changed, and the voltage V2 based on the reference electrode 42 ischanged. Thus, the detection accuracy of the NOx concentration in themeasurement-object gas is decreased. This is different from a temporarychange of the reference potential resulting from, for example, theabove-described reduction of the oxygen concentration in themeasurement-object gas in the periphery of the reference electrode 42and is constant reduction of the sensitivity caused by the degradationof the reference electrode 42. Accordingly, by decreasing the average ofthe current Ip3, it is possible to suppress the degradation of thereference electrode 42 in long time use and to suppress a decrease ofthe detection accuracy of the NOx concentration. The higher the peakcurrent Ip3max, the higher the average of the current Ip3; the longerthe second time T2, the lower the average of the current Ip3. Thus, thepeak current Ip3max and the second time T2 are preferably determined soas to prevent the average of the current Ip3 from being excessivelyhigh.

The second time T2 is preferably 10 msec or less. Note that the longerthe second time T2, the lower the above-described average of the currentIp3. In addition, it is likely that a low average of the current Ip3flowed in the reference electrode 42 results in insufficiency of theeffect of compensating for reduction of the oxygen concentration in theperiphery of the reference electrode 42. As long as the second time T2is not longer than 10 msec, it is likely that insufficiency of theeffect of compensating for reduction of the oxygen concentration in theperiphery of the reference electrode 42 is suppressed.

The fall time Tf is preferably 3 msec or less. Note that it is likelythat the residual voltage Vrs is decreased more rapidly during thesecond period as the fall time Tf is shorter. As long as the fall timeTf is 3 msec or less, it is likely that the second time T2 is maderelatively short or the residual voltage Vrs is sufficiently decreasedduring the second period while maintaining the detection accuracy of theNOx concentration.

As an index of a low residual voltage Vrs, the fall residual voltageDVref10 of the gas sensor 100, calculated according to the followingFormula (2), is preferably 55 mV or less.

DVref10=(Vref2−Vref1)×0.1+Vref1−Vref0   (2),

(where Vref0 is a voltage [mV] between the reference electrode and themeasurement-object gas side electrode in a state in which the layeredbody is placed in an air and in which the control voltage is notapplied,

Vref1 is a minimum voltage [mV] between the reference electrode and themeasurement-object gas side electrode in a state in which the layeredbody is placed in the air and in which the control voltage isrepetitively set to on state and off state, and

Vref2 is a maximum voltage [mV] between the reference electrode and themeasurement-object gas side electrode in a state in which the layeredbody is placed in the air and in which the control voltage isrepetitively set to on state and off state.)

FIG. 4 is an explanatory diagram of the fall residual voltage DVref10.As seen in the above definition and FIG. 4, the voltage Vref2corresponds to the value of the maximum voltage Vrefmax of the voltageVref in FIG. 3 measured in the air. The voltage Vref1 corresponds to thevalue of the minimum voltage Vrefmim during the second period in FIG. 3measured in the air. “(Vref2−Vref1)×0.1+Vref1” in Formula (2)corresponds to a fall voltage V10 a illustrated in FIG. 4. The fallvoltage V10 a corresponds to the value of the fall voltage V10 in FIG. 3measured in the air. The voltage Vref0 corresponds to the value of thevoltage Vref* in FIG. 3 measured in the air. The fall residual voltageDVref10 corresponds to the value of the residual voltage Vrs at a fall,that is, at the start of the second period in FIG. 3 (=fall voltageV10−voltage Vref*) measured in the air. In a case in which the sensorelement 101 is placed in the air, since there is no oxygen concentrationdifference between the periphery of the outer pump electrode 23 and theperiphery of the reference electrode 42, the value of the voltage Vref0is theoretically 0. However, the value of the voltage Vref0 is notactually 0 as a result of a thermoelectromotive force due to atemperature difference between electrodes or the like. Note that since athermoelectromotive force of the same value is also included in the fallvoltage V10, the value of the thermoelectromotive force does not affectthe fall residual voltage DVref10. Technically speaking, theabove-described voltage Vref* in FIG. 3 also includes thethermoelectromotive force. As long as the fall residual voltage DVref10of the gas sensor 100 is 55 mV or less, the fall residual voltage Vrs(fall voltage V10−voltage Vref*) in the measurement-object gas issufficiently low, and accordingly, the lower the residual voltage Vrs atthe fall, the lower the residual voltage Vrs during the second period,and thus, it is likely that the specific gas concentration is detectedwith high accuracy during the second period. The lower the fall residualvoltage DVref10, the more effectively a decrease of the detectionaccuracy of the NOx concentration can be suppressed even if the timingof the detection of the NOx concentration is relatively early in thesecond period.

The voltages Vref0, Vref1, and Vref2 are measured as follows. First, thesensor element 101 is placed in the air, and the heater 72 is powered onto heat the sensor element 101 to a predetermined driving temperature(e.g., 800° C.). No voltage is applied to the variable power supplies25, 46, and 52 and the power supply circuit 92. Then, after thetemperature of the sensor element 101 becomes stable, the voltage Vrefis measured, and the value thereof is set as the voltage Vref0. Then,the control voltage Vp3 is started to be set to on state and off stateby using the power supply circuit 92 so as to start to pump in oxygenfrom the outer pump electrode 23 to the reference electrode 42. Then,the waveform of the voltage Vref one minute after the start of pumpingin is measured, the maximum value of the voltage Vref is set as thevoltage Vref2, and the minimum value thereof is set as the voltageVref1.

The value of the maximum voltage Vp3max can be determined in advance byexperiment or the like so that the peak current Ip3max has a desiredvalue. The reference gas regulation pump cell 90 may detect the peakcurrent Ip3max at a predetermined timing during use of the gas sensor100 and may control the value of the maximum voltage Vp3max on the basisof the detected value. The predetermined timing may be immediately afterthe start of use of the gas sensor 100 or each time a predeterminedperiod elapses during use. The cycle T of the control voltage Vp3 maybe, for example, 5 msec or more and 50 msec or less. The duty ratio thatis the ratio between the cycle T and the on time Ton (Ton/T) may be 0.1or more and 0.8 or less. The off time Toff may be 2 msec or more and 10msec or less. The reference gas regulation pump cell 90 may change atleast any of the cycle T, the duty ratio, and the off time Toff duringuse of the gas sensor 100.

For example, by decreasing the maximum voltage Vrefmax, the residualvoltage Vrs during the second period including the fall voltage V10 canbe generally decreased. Methods for decreasing the maximum voltageVrefmax include decreasing the maximum voltage Vp3max and decreasing theon time Ton. By increasing the off time Toff, the second time T2 can beincreased. The residual voltage Vrs during the second period can begenerally decreased by performing at least one of decreasing the falltime Tf and decreasing the fall voltage V10. As a method for decreasingthe fall time Tf, for example, a time constant τ (=R×C) of the circuitof the reference gas regulation pump cell 90 may be decreased. As amethod for decreasing the time constant τ, for example, an area S of thereference electrode 42 may be increased. As a method for decreasing thefall voltage V10, the maximum voltage Vp3max may be decreased, or theresistance component in the circuit of the reference gas regulation pumpcell 90 may be decreased. As a method for decreasing the resistancecomponent in the circuit of the reference gas regulation pump cell 90,for example, the area S of the reference electrode 42 may be increased.The fall residual voltage DVref10 can be decreased by the same method asthe method for decreasing the fall voltage V10.

The area S of the reference electrode 42 denotes an area of a part ofthe reference electrode 42 facing the air introducing layer 48 and isequal to areas of an upper surface and side surfaces of the referenceelectrode 42 according to this embodiment. The thickness in the verticaldirection of the reference electrode 42 is significantly smaller thanthe length in the front-rear direction and the width in the left-rightdirection of the reference electrode 42, so that the areas of the sidesurfaces (front, rear, left, and right surfaces) of the referenceelectrode 42 are negligible. Therefore, the value of the area S in thisembodiment is the area of the upper surface of the reference electrode42 (length in the front-rear direction×width in the left-rightdirection). As described above, the time constant Z and the resistancecomponent in the circuit of the reference gas regulation pump cell 90can be smaller as the area S is larger, and for this respect, the area Sis preferably 0.4 mm² or more. The area S is more preferably 1.5 mm² ormore. The larger the area S, the smaller the time constant of thecircuit of the reference gas regulation pump cell 90, and the shorterthe fall time Tf. Thus, the residual voltage Vrs during the secondperiod is decreased rapidly. The larger the area S, the smaller theresistance components of the reference electrode 42, and the smaller thevoltage drop due to the current Ip3 flowed in the reference electrode42. Thus, the residual voltage Vrs during the second period isdecreased. Accordingly, the larger the area S, the more rapidly theresidual voltage Vrs during the second period is decreased, and thelower the residual voltage Vrs becomes. Thus, it is likely that the NOxconcentration is detected with high accuracy during the second period.An upper limit of the area S can be determined as appropriate inaccordance with, for example, the size of the sensor element 101. Thearea S may be not greater than 5 mm². The length in the front-reardirection, the width in the left-right direction, and the thickness ofthe reference electrode 42 are not specifically limited, but the lengthin the front-rear direction may be, for example, 0.2 to 2 mm, the widthin the left-right direction may be, for example, 0.2 to 2.5 mm, and thethickness may be, for example, 5 to 30 Lm.

As in the measurement pump cell 41, operations of the main pump cell 21and the auxiliary pump cell 50 are preferably performed in each cycle Tand during the second period. For example, the main pump cell 21preferably acquires the electromotive force V0 and performs feedbackcontrol of the pump voltage Vp0 on the basis of the acquiredelectromotive force V0 in each cycle T and during the second period. Theauxiliary pump cell 50 preferably acquires the electromotive force V1and performs feedback control of the pump voltage Vp1 based on theacquired electromotive force V1 in each cycle T and during the secondperiod. Thus, the operations of these cells are unlikely to be affectedby a change in the potential of the reference electrode 42 caused by thecontrol voltage Vp3.

The following gives the correspondence relationship between thecomponents according to this embodiment and components according to thepresent invention. The first substrate layer 1, the second substratelayer 2, the third substrate layer 3, the first solid electrolyte layer4, the spacer layer 5, and the second solid electrolyte layer 6according to this embodiment correspond to a layered body according tothe present invention; the reference electrode 42 corresponds to areference electrode; the outer pump electrode 23 corresponds to ameasurement-object gas side electrode; the air introducing layer 48corresponds to a reference gas introducing layer; the reference gasregulation pump cell 90 corresponds to a reference gas regulatingdevice; and the measurement pump cell 41 corresponds to a detectingdevice. The outer pump electrode 23 corresponds to an outer electrode.

In the gas sensor 100 of the embodiment specifically described above,the reference gas regulation pump cell 90 applies the control voltageVp3 so as to pump in oxygen to the periphery of the reference electrode42. This compensates for reduction of the oxygen concentration in theperiphery of the reference electrode 42. Further, the reference gasregulation pump cell 90 also applies the control voltage Vp3 that isrepetitively set to on state and off state, and the measurement pumpcell 41 detects the NOx concentration on the basis of the voltage V2during the second period. This suppresses a decrease of the detectionaccuracy of the NOx concentration caused by the control voltage Vp3.Thus, the gas sensor 100 can pump in oxygen to the periphery of thereference electrode 42 and can suppress a decrease of the detectionaccuracy of the NOx concentration caused by the control voltage Vp3 usedfor pumping in.

In addition, as long as the peak current Ip3max is 10 μA or more, it islikely that the gas sensor 100 sufficiently compensates for reduction ofthe oxygen concentration in the periphery of the reference electrode 42.As long as the second time T2 is not longer than 10 msec, it is likelythat insufficiency of an effect of compensating for reduction of theoxygen concentration in the periphery of the reference electrode 42 issuppressed. As long as the fall time Tf is 3 msec or less, it is likelythat the second time T2 is made relatively short or the residual voltageVrs is sufficiently decreased during the second period while maintainingthe detection accuracy of the NOx concentration. As long as the fallresidual voltage DVref10 is 55 mV or less, it is likely that the gassensor 100 detects the NOx concentration with high accuracy during thesecond period.

The present invention is not limited to the embodiment described abovebut may be implemented by a diversity of other configurations withoutdeparting from the technical scope of the invention.

In the above-described embodiment, the air introducing layer 48 isprovided from the reference electrode 42 to the rear end surface of thesensor element 101 in the longitudinal direction, but this is notrestrictive. FIG. 5 is a sectional schematic diagram of a sensor element201 according to a modification of this case. As illustrated in FIG. 5,the sensor element 201 includes a reference gas introducing space 43above an air introducing layer 248. The reference gas introducing space43 is a space provided between an upper surface of the third substratelayer 3 and a lower surface of the spacer layer 5. Also, side portionsof the reference gas introducing space 43 are defined by side surfacesof the first solid electrolyte layer 4. A rear end of the reference gasintroducing space 43 is open to a rear end surface of the sensor element201. The reference gas introducing space 43 is provided to be ahead ofthe pressure release hole 75 in the front-rear direction, and thepressure release hole 75 is open to the reference gas introducing space43. Unlike the air introducing layer 48, the air introducing layer 248is not provided to a rear end of the sensor element 201. Thus, the airintroducing layer 248 is not exposed to the rear end surface of thesensor element 201. Instead, a part of an upper surface of the airintroducing layer 248 is exposed to the reference gas introducing space43. The exposed portion serves as the inlet portion 48 c of the airintroducing layer 248. The reference gas is introduced to the airintroducing layer 248 from the inlet portion 48 c through the referencegas introducing space 43. Note that a rear end of the air introducinglayer 248 may be provided to the rear end of the sensor element 201 inthe sensor element 201.

In the above-described embodiment, the sensor element 101 of the gassensor 100 includes the first internal cavity 20, the second internalcavity 40, and the third internal cavity 61, but this is notrestrictive. For example, as in the above-described sensor element 201illustrated in FIG. 5, the third internal cavity 61 may be omitted. Inthe sensor element 201 according to the modification illustrated in FIG.5, the gas inlet port 10, the first diffusion controlling portion 11,the buffer space 12, the second diffusion controlling portion 13, thefirst internal cavity 20, the third diffusion controlling portion 30,and the second internal cavity 40 are formed between the lower surfaceof the second solid electrolyte layer 6 and the upper surface of thefirst solid electrolyte layer 4 to be adjacent to one another andcommunicate with one another in this sequence. The measurement electrode44 is provided on the upper surface of the first solid electrolyte layer4 in the second internal cavity 40. The measurement electrode 44 isformed by being covered by a fourth diffusion controlling portion 45.The fourth diffusion controlling portion 45 is a film made of a ceramicporous material such as alumina (Al₂O₃). The fourth diffusioncontrolling portion 45 serves to limit the amount of NOx flowing intothe measurement electrode 44 as in the fourth diffusion controllingportion 60 according to the above-described embodiment. In addition, thefourth diffusion controlling portion 45 also serves as a protective filmof the measurement electrode 44. The top electrode portion 51 a of theauxiliary pump electrode 51 is formed immediately above the measurementelectrode 44. The sensor element 201 having such a configuration canalso detect the NOx concentration by using the measurement pump cell 41as in the above-described embodiment.

Note that, in the sensor element 201 illustrated in FIG. 5, nomodification may be made on the reference gas introducing space 43 andthe air introducing layer 248, and the fourth diffusion controllingportion 60 and the third internal cavity 61 may be provided as in theabove-described embodiment. In addition, in the sensor element 101according to the above-described embodiment, no modification may be madeon the fourth diffusion controlling portion 60 and the third internalcavity 61, and the same configuration as the reference gas introducingspace 43 and the air introducing layer 248 illustrated in FIG. 5 may beemployed.

In the above-described embodiment, the outer pump electrode 23 servingas the outer electrode of the measurement pump cell 41 also serves asthe measurement-object gas side electrode of the reference gasregulation pump cell 90, but this is not restrictive. The outerelectrode of the measurement pump cell 41 and the measurement-object gasside electrode of the reference gas regulation pump cell 90 may beformed separately on the outer surface of the sensor element 101. Inaddition, as long as the measurement-object gas side electrode of thereference gas regulation pump cell 90 is provided in a portion exposedto the measurement-object gas in the sensor element 101, the providingposition is not limited to the outer surface. For example, themeasurement-object gas side electrode may be provided in themeasurement-object gas flowing portion.

In the above-described embodiment, the voltage Vp2 of the variable powersupply 46 is controlled to maintain constant the voltage V2 detected bythe measurement pump-controlling oxygen partial pressure detectionsensor cell 82, and the concentration of nitrogen oxides in themeasurement-object gas is calculated by using the pump current Ip2 underthe control. This is, however, not restrictive as long as the specificgas concentration in the measurement-object gas is detected on the basisof the voltage between the reference electrode 42 and the measurementelectrode 44. For example, the measurement electrode 44, the first solidelectrolyte layer 4, the third substrate layer 3, and the referenceelectrode 42 may be combined to constitute an electrochemical sensorcell serving as an oxygen partial pressure detecting device. Thiselectrochemical sensor cell is capable of detecting a voltage accordingto a difference between the amount of oxygen produced by reduction ofthe NOx component in the ambient atmosphere of the measurement electrode44 and the amount of oxygen in the periphery of the reference electrode42 and thereby determines the concentration of the NOx component in themeasurement-object gas. In this case, this electrochemical sensor cellcorresponds to the detecting device of the present invention.

The reference electrode 42 is formed directly on the upper surface ofthe third substrate layer 3 in the above-described embodiment, but thisis not restrictive. For example, the reference electrode 42 may beformed directly on the lower surface of the first solid electrolytelayer 4.

The reference gas is the air in the above-described embodiment. Thereference gas is, however, not limited to this but may be any gas thatcan be used as a standard for detection of a specific gas concentrationin the measurement-object gas. For example, the space 149 may be filledwith a gas with an oxygen concentration adjusted in advance to apredetermined value (> oxygen concentration in the measurement-objectgas) as the reference gas.

The sensor element 101 detects the NOx concentration in themeasurement-object gas in the above-described embodiment, but this isnot restrictive. The sensor element may detect any specific gasconcentration in the measurement-object gas, for example, the oxygenconcentration in the measurement-object gas.

EXAMPLES

The following describes concrete examples of manufacturing gas sensorsas examples. Experimental Examples 1 to 9, 11, 12, 14, and 16 to 22 areexamples of the present invention. Experimental Examples 10, 13, and 15are comparative examples. The present invention is, however, not limitedto the following examples.

Experimental Example 1

The gas sensor 100 illustrated in FIGS. 1 and 2 was produced by themanufacturing method described above as Experimental Example 1. Theceramic green sheets used for production of the sensor element 101 wereformed by tape casting of a mixture of zirconia particles containing 4mol % yttria as a stabilizing agent with an organic binder and anorganic solvent. The green compacts 145 a and 145 b illustrated in FIG.1 were compacted talc powder. The air introducing layer 48 was made ofceramic of alumina. The porosity of the air introducing layer 48 was 40volume %. The area S of the reference electrode 42 was 0.6 mm². Thecontrol voltage Vp3 applied by the power supply circuit 92 of thereference gas regulation pump cell 90 was a pulse voltage with the cycleT being 10 msec, the on time Ton being 2.0 msec, and the off time Toffbeing 8.0 msec. The maximum of the control voltage Vp3 (maximum voltageVp3max) applied by the power supply circuit 92 was set to a value withwhich the peak current Ip3max flowed in the reference electrode 42 byusing the control voltage Vp3 became 22 IA. The fall time Tf of thevoltage Vref of the sensor cell 83 based on the control voltage Vp3 was1.5 msec, and the second time T2 was 6.5 msec.

Experimental Examples 2 to 22

Gas sensors of Experimental Examples 2 to 22 were produced in the samemanner as Experimental Example 1 for the sensor element 101, except thatthe area S of the reference electrode 42, the cycle T, the on time Ton,the off time Toff, and the peak current Ip3max were changed in variousmanners as illustrated in Table 1. The peak current Ip3max was adjustedby changing the maximum voltage Vp3max. Thus, in Experimental Examples 2to 22, the fall time Tf, the second time T2, and the ratio T2/Tf werechanged in various manners from Experimental Example 1.

[Evaluation of Detection Accuracy]

The gas sensor of Experimental Example 1 was mounted to an exhaust gaspiping of an automobile. The heater 72 was then powered on to heat thesensor element 101 at a temperature of 800° C. Subsequently, a gasolineengine (1.8 L) of the automobile was operated under predeterminedconditions (rotation speed of the engine was 4500 rpm, the air/fuelratio A/F was 11.0, the load torque was 130 N-m, the gauge pressure ofexhaust gas was 60 kPa, and the temperature of exhaust gas was 800° C.).In this state, all the pump cells including the reference gas regulationpump cell 90 were operated, the measurement of the NOx concentration wasstarted, and the reference gas regulation pump cell 90 started to pumpin oxygen to the reference electrode 42. The measurement pump cell 41detected the NOx concentration at a timing as later as possible duringthe second period in each cycle T. The pump cells other than themeasurement pump cell 41 and the reference gas regulation pump cell 90were also operated in the same cycle as the cycle T, and the operationperiod was a timing as later as possible during the second period.Immediately after 80 seconds, which is necessary for initialstabilization of the pump cells, elapsed from the start of the operationof all the pump cells, the pump current Ip2 was measured. The pumpcurrent Ip2 for Experimental Examples 2 to 22 was measured in the samemanner. For each of Experimental Examples 1 to 22, if the measured pumpcurrent Ip2 falls within a first allowable range (within a range of 50%to 200% with a correct value being 100%) that is very close to a correctvalue (value corresponding to a NOx concentration of 500 ppm in theexhaust gas), the detection accuracy of the NOx concentration wasdetermined as “A”. If the measured pump current Ip2 is beyond the firstallowable range, the detection accuracy of the NOx concentration wasdetermined as “B”.

Table 1 shows the area S, the peak current Ip3max, the cycle T, the ontime Ton, the off time Toff, the fall time Tf, the second time T2, theratio T2/Tf, and the evaluation results of the detection accuracy inExperimental Examples 1 to 22. Table 1 also shows the evaluation resultsof the pump-in amount of oxygen, which will be described later, inExperimental Examples 1 to 22.

TABLE 1 Evaluation Area S of Peak Voltage Vp3 Voltage Vref Evaluationresults of reference current On time Off time Second results of pump-inelectrode Ip3max Cycle T Ton Toff Fall time Tf time T2 Ratio detectionamount of [mm²] [μA] [msec] [msec] [msec] [msec] [msec] T2/Tf accuracyoxygen Experimental 0.6 22 10 2 8 1.5 6.5 4.31 A A example 1Experimental 0.6 11 10 2 8 1.4 6.6 4.69 A A example 2 Experimental 0.6110 10 2 8 1.6 6.4 3.98 A A example 3 Experimental 0.6 100 10 2 8 1.76.3 3.68 A A example 4 Experimental 0.4 50 10 3 7 1.9 5.1 2.66 A Aexample 5 Experimental 1.8 27 10 2 8 1.3 6.7 5.13 A A example 6Experimental 0.4 10 10 2 8 2.0 6.0 2.98 A A example 7 Experimental 0.4120 10 3 7 2.0 5.0 2.48 A A example 8 Experimental 0.6 150 10 2 8 1.56.5 4.31 A A example 9 Experimental 0.4 44 5 2 3 2.0 1.0 0.48 B Aexample 10 Experimental 3.0 30 5 2 3 0.5 2.5 4.98 A A example 11Experimental 0.6 18 5 2 3 1.4 1.6 1.11 A A example 12 Experimental 0.355 10 5 5 3.0 1.9 0.64 B A example 13 Experimental 0.3 2 20 5 15 2.412.5 5.23 A B example 14 Experimental 0.1 33 10 3 7 5.0 1.9 0.38 B Aexample 15 Experimental 0.3 40 20 3 17 3.0 13.9 4.64 A A example 16Experimental 0.1 50 20 5 15 5.0 9.9 1.97 A A example 17 Experimental 0.69 10 3 7 1.8 5.2 2.87 A B example 18 Experimental 0.2 100 20 2 18 4.013.9 3.48 A A example 19 Experimental 0.4 55 20 10 10 2.0 8.0 3.98 A Aexample 20 Experimental 1.8 11 10 2 8 1.0 8.0 7.95 A A example 21Experimental 1.8 100 10 2 8 1.4 8.0 5.68 A A example 22

As shown in Table 1, in each of Experimental Examples 1 to 9, 11, 12,14, and 16 to 22 in which the above Formula (1) is satisfied, that is,the ratio T2/Tf is 1 or more, the evaluation of the detection accuracyis A. On the other hand, in Experimental Examples 10, 13, and 15 inwhich the ratio T2/Tf is less than 1, the evaluation of the detectionaccuracy is B. These results show that the ratio T2/Tf is preferably 1or more.

[Evaluation of Pump-in Amount of Oxygen]

As in the above-described detection accuracy evaluation test, the gassensor of Experimental Example 1 was mounted to a piping, and then theheater 72 was powered on. Subsequently, a gasoline engine of theautomobile was operated under the same conditions as those of theabove-described detection accuracy evaluation test, and only thereference gas regulation pump cell 90 was operated. This state wasmaintained for 20 minutes, during which the voltage Vref was measured todetermine whether the minimum voltage Vrefmin was decreased to be lowerthan a predetermined threshold. The threshold was set to 70% of thevalue at the time of starting measurement of the minimum voltageVrefmin. If the minimum voltage Vrefmin did not become lower than thethreshold after 20 minutes, the pump-in amount of oxygen was determinedas “A”. If the minimum voltage Vrefmin became lower than the thresholdbefore 20 minutes, the pump-in amount of oxygen was determined as “B”.The minimum voltage Vrefmin for Experimental Examples 2 to 22 wasmeasured in the same manner for evaluation. Note that the operation ofthe reference gas regulation pump cell 90 supplies the control currentIp3 and pumps in oxygen, so as to compensate for reduction of the oxygenconcentration in the periphery of the reference electrode 42 due topenetration of the exhaust gas into the space 149. On the other hand, asthe oxygen concentration of the reference electrode 42 is decreased, theoxygen concentration difference between the outer pump electrode 23 andthe reference electrode 42 is reduced to decrease the voltage Vref*, andthereby the minimum voltage Vrefmin is also decreased. Accordingly, ahigh-speed decrease of the minimum voltage Vrefmin over time means thatthe pump-in amount of oxygen by the reference gas regulation pump cell90 is insufficient and that the effect of compensating for reduction ofthe oxygen concentration is insufficient. The above Table 1 shows theevaluation results of the pump-in amount of oxygen in ExperimentalExamples 1 to 22.

As shown in Table 1, in each of Experimental Examples 1 to 13, 15 to 17,and 19 to 22 in which the peak current Ip3max is 10 μA or more, theevaluation of the pump-in amount of oxygen is A. On the other hand, inExperimental Examples 14 and 18 in which the peak current Ip3max is lessthan 10 μA, the evaluation of the pump-in amount of oxygen is B. Theseresults show that the peak current Ip3max is preferably 10 μA or more.

The present application claims priority from Japanese Patent ApplicationNo. 2017-070702, filed on Mar. 31, 2017, the entire contents of whichare incorporated herein by reference.

What is claimed is:
 1. A gas sensor comprising: a layered body that isformed by stacking a plurality of oxygen ion-conductive solidelectrolyte layers, and that includes a measurement-object gas flowingportion which a measurement-object gas is introduced and flowed in; areference electrode that is formed inside of the layered body, and thatreceives a reference gas introduced therein, the reference gas beingused as a standard for detection of a specific gas concentration in themeasurement-object gas; a measurement electrode provided on an innerperipheral surface of the measurement-object gas flowing portion; ameasurement-object gas side electrode provided in a region of thelayered body that is exposed to the measurement-object gas; a referencegas introducing portion that introduces the reference gas to a peripheryof the reference electrode; a reference gas regulating device thatapplies a control voltage that is repetitively set to on state and offstate between the reference electrode and the measurement-object gasside electrode to pump in oxygen to the periphery of the referenceelectrode; and a detecting device that detects the specific gasconcentration in the measurement-object gas on the basis of a voltagebetween the reference electrode and the measurement electrode during asecond period, from among a first period that is started upon setting ofthe control voltage to on state, during which a potential differencebetween the reference electrode and the measurement-object gas sideelectrode is large, and the second period that is started upon settingof the control voltage to off state after the potential difference fallsfrom the first period, wherein the gas sensor satisfies the followingFormula (1),Tf≤T2  (1) (where Tf is a fall time [msec] of the potential differencebetween the first period and the second period, and T2 is a second time[msec] that is a length of the second period).
 2. The gas sensoraccording to claim 1, wherein the peak current Ip3max flowed in thereference electrode by using the control voltage is 10 μA or more. 3.The gas sensor according to claim 1, wherein the second time T2 is 10msec or less.
 4. The gas sensor according to claim 1, wherein the falltime Tf is 3 msec or less.
 5. The gas sensor according to claim 1,wherein a fall residual voltage DVref10 calculated according to thefollowing Formula (2) is 55 mV or less,DVref10=(Vref2−Vref1)×0.1+Vref1−Vref0  (2), (where Vref0 is a voltage[mV] between the reference electrode and the measurement-object gas sideelectrode in a state in which the layered body is placed in an air andin which the control voltage is not applied, Vref1 is a minimum voltage[mV] between the reference electrode and the measurement-object gas sideelectrode in a state in which the layered body is placed in the air andin which the control voltage is repetitively set to on state and offstate, and Vref2 is a maximum voltage [mV] between the referenceelectrode and the measurement-object gas side electrode in a state inwhich the layered body is placed in the air and in which the controlvoltage is repetitively set to on state and off state.)